Here’s good article about using CSS text replacement (FIR) technique with Google and SEO.

Looks like, as long as you don’t use long strings of keywords when hiding text, you should be ok.

Quote from Chris a Google.com:

If you’re straight-out using CSS to hide text, don’t be surprised if that is called spam. I’m not saying that mouseovers or DHTML text or have-a-logo-but-also-have-text is spam; I answered that last one at a conference when I said “imagine how it would look to a visitor, a competitor, or someone checking out a spam report. If you show your company’s name and it’s Expo Markers instead of an Expo Markers logo, you should be fine. If the text you decide to show is ‘Expo Markers cheap online discount buy online Expo Markers sale …’ then I would be more cautious, because that can look bad.